Potassium Management Effects on Chloride Cycling and Potato Yield and Quality

Potato production requires large amounts of potassium (K), without which low yield, poor bulking, and other issues may arise. Due to a recent, prolonged peak in K prices, there is strong interest in whether banded K application could reduce K fertilizer requirements in potato crops. Alternatively, K use efficiency might be improved by using split applications of K, reducing losses of K to fixation and leaching before the crop is done taking it up. There are also concerns that fertilization with potassium chloride (KCl), which is affordable and therefore widely used, may result in reduced tuber specific gravity, and whether this effect is due to excessive chloride (Cl) application. The objectives of this study were to (1) evaluate the effects of K rate on tuber yield and quality, (2) determine whether banded K application decreases potato crop K requirements, (3) evaluate the effectiveness of split K application in improving K use efficiency, (4) determine whether using K2SO4 in place of KCl improves tuber specific gravity, and (5) evaluate the effects of Cl application on potato crop performance and Cl leaching concentrations. Among broadcast KCl treatments, the highest total yield and specific gravity were observed at 80 lbs/ac K2O, while the highest yields of U.S. No. 1 and total marketable potatoes were seen at 160 lbs/ac K2O. Total and marketable yields tended to decrease as broadcast rates increased above 160 lbs/ac as K2O. Banded application of K increased the yield of U.S. No. 2 tubers, but decreased tuber specific gravity. Split application of KCl produced significantly higher total, marketable, and U.S. No. 1 yields than a single broadcast application at the same total rate (240 lbs/ac K2O). Using potassium sulfate (K2SO4) in place of KCl did not improve tuber specific gravity, nor did a treatment receiving calcium chloride without K have lower tuber specific gravity than the check treatment, suggesting that Cl- did not reduce specific gravity in this study. At equivalent K2O rates (160 or 240 lbs/ac), yields with K2SO4 were higher than KCl. Banded application of KCl increased the concentration of Cl- in soil water sampled at the 4 ft depth compared to broadcast applications of KCl or K2SO4 at the same K rate. It is possible that banded applications would have produced better yield and less Cl- leaching if applied at lower rates than broadcast applications. The negative effect of high KCl rates was likely due to lack of rainfall/leaching with low Cl- concentrations during the growing season coupled with high concentrations of Cl- in irrigation water, which supplied more than 150 lbs/ac Cl- over the growing season.